When starting from rest or low speeds, a boat designed to operate in a planing mode must make a transition from conditions in which a large part of the hull is immersed in the water (under which conditions, the boat generates a large wave/displacement drag) to a situation in which the hydrodynamic forces on the hull of the boat caused by its motion through the water cause much of the vessel to be raised out of the water, thereby reducing drag and allowing the attainment of high speeds under planing conditions.
Some craft exhibit an undesirable attitude in the water which prevents or limits the rapid achievement of this transition. This can be due to a variety of reasons; for example, the craft may be stern heavy or the craft may be towing a water skier from rest, thereby pulling down the stern of the craft. If the boat is under-powered, it may be unable to make the transition to planing effectively, or it may take considerable time to do so.
To overcome these problems, wing-type lift-generating devices are often fitted to boats, typically to the submerged part of the propulsion unit. Flow-deflecting plates are also frequently placed on the rear of the transom of a boat.
The intention of both these types of equipment is to increase the lift force at the stern of the boat. This partly increases the overall lift on the hull but, more importantly, changes the attitude of the boat in order to reduce the drag at lower speeds, thereby allowing an easier and more rapid transition to the planing mode.
Such devices are frequently employed, the wing-type operating in the same manner as rather inefficient aircraft wings. The deflector plates produce lift by changing the direction of the flow caused by the motion of the boat in the same way that the planing hull of the boat creates lift. Both wings and deflectors require movement of the boat through the water in order to function and, with both types of device, the lift increases with increasing speed of the boat through the water. Both types of device are thus least effective when lift at the stern is most needed, i.e., when the boat is stationery or is moving very slowly. In fact, it could be advantageous to have the reverse lift/speed relationship, i.e. for the lift produced by a lift-generating device to be at a maximum when the boat is at rest and either stay constant or reduce in some manner as the boat approaches its operational speed.
It is accordingly an object of the present invention to provide an improved form of lift-generating device for a boat, particularly one which provides maximum lift when the boat is at rest or moving slowly through the water.